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Please quote the Greek text you are discussing directly in your post if it is reasonably short - do not ask people to look it up. This is not a beginner's forum, competence in Greek is assumed.

Perhaps the addition of the extra phrase ἀμφότεροι οἱ καρποὶ τῶν χειρῶν αὐτοῦ is a marginal interpolation, or perhaps it is a way of clarifying that the "wrists" were not meant, but what other "wrists" are there? Why is הַמִּפְתָּ֔ן lexically untranslated? The phrase ἐπὶ τὰ ἐμπρόσθια αμαφεθ seems like a clumsy back translation of ἐπὶ τὸ πρόθυρον.

The entire section seems badly and unidiomatically translated. Do others have a different take on it?

It's sometimes very difficult to get inside the head of a LXX translator and figure out why he translated the way he did. We used to have Ken Penner and a few other Septuagintalists posting here? However, I will point out that 1 Reigns 5:4 is not 1 Kings 5:4 in the English Bible, but 1 Samuel 5:4, just to avoid confusion.

Are translations of the Hebrew
וּשְׁתֵּ֣י כַּפּ֣וֹת יָדָ֗יו כְּרֻתוֹת֙ אֶל־הַמִּפְתָּ֔ן

Albeit the first one isn't nearly as good as the second.

Either that, or when translating the translator skipped back and did the same phrase again, though that seems unlikely.

Could argue that τὰ ἴχνη χειρῶν αὐτοῦ and οἱ καρποὶ τῶν χειρῶν αὐτοῦ are referring to different sections of the 'hand' - ἴχνη χειρῶν/feet of the hands = upper forearm ; καρποὶ τῶν χειρῶν/fruit of the hands = main palm.